Between 1920 and 1935, mathematicians and experimentalists began laying the groundwork for a theory combining Darwinian evolution and Mendelian genetics. Starting his career about this time, Dobzhansky was involved in the project almost from its inception. His book Genetics and the Origin of Species(1937) was the first substantial synthesis of the subjects and established evolutionary genetics as an independent discipline. Until the 1930s, the commonly held view was that natural selection produced something close to the best of all possible worlds and that changes would be rare and slow and not apparent over one life span, in agreement with the observed constancy of species over historical time.

Dobzhansky’s most important contribution was to change this view. In observing wild populations of the vinegar fly Drosophila pseudoobscura, he found extensive genetic variability. Furthermore, about 1940 evidence accumulated that in a given local population some genes would regularly change in frequency with the seasons of the year. For example, a certain gene might appear in 40 percent of all individuals in the population in the spring, increase to 60 percent by late summer at the expense of other genes at the same locus, and return to 40 percent in overwintering flies. Compared to a generation time of about one month, these changes were rapid and effected very large differences in reproductive fitness of the various types under different climatic conditions. Other experiments showed that, in fact, flies of mixed genetic makeup (heterozygotes) were superior in survival and fertility to pure types.

It was already known that these superiorities of such heterozygotes would ensure the preservation of both sets of genes in the population. Dobzhansky pointed out that newly arisen genes are rare at first and that an individual is exceedingly unlikely to receive such a gene from both parents. Hence, in the beginning, the only genes that can “get ahead” and become more widespread in the population are those that are “good mixers”—that is, those that produce superior genotypes when combined with a random gene from the population.

A genetic system of the kind proposed by Dobzhansky can change rapidly, in response to natural selection, if environmental conditions should change. Among the myriad genotypes appearing in each generation would be many that were adapted to the changed conditions and that would leave more descendants; thus, these genes would be more common in the next generation. In contrast, under the older idea of a fairly uniform population in which most gene variants occurred rarely, much more time would be needed before variants adapted to new conditions could arise and become common. Meanwhile, local populations of the species could be in danger of becoming very reduced in numbers or even extinct.

Other important work of Dobzhansky dealt with speciation: the process by which a species does not merely change its characteristics over time but actually splits into two or more species. In extension of his work in human genetics and in human paleontology, Dobzhansky also wrote on the “descent of man” in Mankind Evolving (1962). Finally, his interest in the direction that human evolution might take in the future, added to a natural philosophical inclination, led him into thought on the nature of humans and the purpose of life and death, as shown in his works The Biological Basis of Human Freedom (1956) and The Biology of Ultimate Concern (1967). Genetics of the Evolutionary Process (1970) reflects 33 years of scientific progress in the study of evolution, largely by Dobzhansky or under his influence.

Although preeminently a laboratory biologist and writer, Dobzhansky never lost his liking for fieldwork; he boasted of having collected specimens from Alaska to Tierra del Fuego and in every continent except Antarctica. An inspiring teacher and lecturer, he received over the years a steady stream of scientists from other countries, who came to spend time in his laboratory to learn his approach to research.

Beginning in 1918, Dobzhansky published well over 400 research papers that provide an important part of the factual evidence for modern evolutionary theory. His preeminence, however, lay even more in the rare talent for synthesizing the masses of experimental and theoretical data in the literature into a broad, comprehensive view of the subject.

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A major breakthrough came in 1937 with the publication of Genetics and the Origin of Species by Theodosius Dobzhansky, a Russian-born American naturalist and experimental geneticist. Dobzhansky’s book advanced a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical argument. ...

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(1900-75). A Russian American scientist, Theodosius Dobzhansky had a major influence on 20th-century thought and research in genetics and the study of evolution. From 1918 he published more than 400 research papers that provide important factual evidence for modern evolutionary theory.